The free-piston linear engine is a twentieth century invention that eliminates the need for piston rods and a crankshaft, which are traditionally associated with internal combustion piston engines. Within a free-piston engine, power may be provided to a load by devices other than a crankshaft, e.g., hydraulically or by an exhaust gas turbine. The engine is referred to as a “free” piston engine because the piston travels freely back and forth within the combustion cylinder(s). Free-piston engines have been used as air compressors, hydraulic power devices, and as linear motor/generators.
One of the advantages of free-piston engines is that they may employ higher compression ratios than the compression ratios typically associated with crankshaft engines. Because of this characteristic, free-piston engines may efficiently utilize a wide spectrum of different ignition regimes and fuels. Free-piston engines have been built, or have been proposed, to operate with spark ignition, diesel ignition, and homogenous change compression ignition for example. The free-piston engine's high compression ratio capability, coupled with the flexibility to utilize different ignition regimes, suggests that this engine may revolutionize 21st century combustion engine technology.
Recently, the work of Dr. Peter Van Blarigan at Sandia National Laboratories has demonstrated that a free-piston engine can effectively operate with homogenous charge compression ignition (HCCI). See, U.S. Pat. No. 6,199,519. An HCCI engine represents an important advance in the state of the art because it allows combustion efficiency to approach ideal Otto cycle efficiency. HCCI is an extremely high compression ratio ignition regime. Fuel and air are pre-mixed in the combustion cylinder (unlike the diesel cycle where the air is first heated by compression and the fuel is then injected in a droplet form). During compression of the fuel/air charge, the temperature in the cylinder reaches, or exceeds, the autoignition temperature of the selected fuel in air. Consequently, the fuel/air charge spontaneously ignites when the appropriate compression ratio and temperature is attained in the cylinder. Extremely lean fuel/air mixtures are possible with HCCI. The free-piston engine's unique ability to achieve extremely high compression ratios makes it an ideal engine for HCCI operation.
The Van Blarigan free-piston engine is a two-stroke, linear motor/generator. It has dual combustion chambers, one on either side of the moveable free-piston. The exterior surface of the piston contains magnets, although in some embodiments the piston may serve as a platform for a shutter, as discussed below.
After the HCCI combustion event in the first combustion chamber, the piston is propelled toward the second combustion chamber. The movement of the magnets, located on the piston's surface, induces an electrical current in coils located in a stator that surrounds the piston. The free-piston provides the necessary force to compress the fuel/air charge in the second combustion chamber to its autoignition temperature. The combusted gases in the first combustion chamber are scavenged, by means of an external device (e.g., a turbocharger), during this stroke and a fresh charge of fuel and air is introduced. After ignition in the second combustion chamber, the piston travels back toward the first chamber to compress and ignite the fresh fuel/air charge. An electric current is again generated as the magnets on the piston's surface traverse the surrounding coils. The combusted gases from the second combustion chamber are scavenged and a fresh fuel/air charge is introduced into that chamber.
The two-stroke cycle then repeats. Van Blarigan reports high combustion efficiencies with HCCI combustion. For example, with methane as the fuel, Van Blarigan has found that thermal efficiencies of over 50% can be achieved, which approaches ideal Otto cycle efficiency.
In U.S. Pat. No. 6,541,875, Berlinger has disclosed a free-piston linear motor/generator in which the magnetic armature and coils are physically separated from the combustion chamber. Physical separation of the magnets and coils is possible because the Berlinger engine employs a single combustion chamber, rather than the dual combustion chamber of the Van Blarigan engine. This physical separation of the magnets and the coils from the single combustion chamber reduces the likelihood that the magnets and/or coils will become overheated during engine operation. However, the Berlinger single combustion chamber design is less efficient than the dual combustion chamber engine because electricity must be used to return the piston from its BDC position to its TDC position. In the dual combustion chamber design, such as the Van Blarigan engine, the momentum of the piston is preferably used to move the piston from its BDC position to its TDC position, thereby increasing the engine's overall mechanical efficiency. The Berlinger engine also relies on the linear motion of the piston to generate electrical power.
In the Wechner dual combustion chamber linear motor/generator, liquid cooling is employed in an area adjacent to the coils and the fresh intake air is drawn through the main body of the piston in order to provide a degree of air-cooling to the magnets located circumferentially on the exterior of the piston. See, U.S. Pat. No. 6,651,599. The Wechner motor/generator's piston is substantially hollow in order to allow for the delivery of compressed air for scavenging. Like the Van Blarigan and Berlinger devices, the Wechner engine locates the magnets on the surface of the piston, which adds to piston weight and reduces linear piston velocity.
Because the voltage produced by a free piston generator is proportional to the speed of the piston, it is advantageous to minimize the piston weight as much as possible in motor/generator designs which solely rely upon linear motion. Removal of the magnets from the piston main body and positioning the magnets on the stator allows a significant reduction in piston weight. Southwest Research Institute (SWRI) has modeled a single chamber free-piston motor/generator in which the magnets and coils are located on the stator. The SWRI model is cited by Van Blarigan as an alternate free-piston linear motor/generator configuration. SWRI has identified two configurations in which high electrical efficiency may be achieved with linear piston motion and with the magnets and coils mounted on the stator of the engine. The first design is a two-coil linear inductance generator. The magnets generate a magnetic field around the armature windings. The piston has a sleeve, which, as the piston moves from top dead center (TDC) to bottom dead center (BDC), acts as a shutter to collapse the magnetic field. As the shutter crosses the magnetic field, the field collapses and a current is induced in the armature. As the piston returns to TDC from BDC, the magnetic field is restored. SWRI reports that the two-coil inductance generator may be 90% electrically efficient. In a second design, a direct current may be generated as the shutter moves through a magnetic field produced by a ferromagnetic yoke and field windings. A sliding contact is required by this configuration. SWRI estimates that this design may also be 90% electrically efficient. Unfortunately, the relatively large air gap associated with shutters limits the actual electrical power that can be derived from this model. Also, because the SWRI model is a single chamber design, the piston is returned to TDC by means of a bounce cylinder, which is less efficient than the dual chamber configuration. The SWRI model is further based on spark ignition, rather than high compression diesel ignition or HCCI.
The SWRI inventors note that there are several significant operating deficiencies associated with free-piston linear generators that mount the magnets on the piston. Among those concerns are exposure of the magnets to high temperatures, high vibration levels or both. In addition, the weight of permanent magnets directly affect the operational speed of the reciprocating free-piston. The operational speed of the piston is critically dependent on piston weight. Because the induced voltage of the free-piston generator is proportional to piston velocity, by mounting heavy permanent magnets on the piston, the linear velocity of the piston will be reduced and the voltage-producing capability of the engine will be adversely impacted.
The art of free-piston technology further discloses that, unlike the above-cited devices, in which the piston's motion is limited to linear motion, a free-piston engine can operate as a rotary engine. This approach is typified by the General Motors free-piston gas turbine engine developed in the 1950's. The General Motors engine utilized the exhaust gas from two free-piston engines in order to turn a gas turbine, which was the primary load. The exhaust gas effectively converted the linear motion of the free-piston engine into rotary motion. However, delivery of exhaust gas through a receiver lowered the overall system efficiency. Moreover, the usable exhaust gas pressure from the free-piston engines was limited by the temperatures that could be tolerated by the blades of the gas turbine.
The patent literature discloses several other devices that have been previously employed to convert the linear motion of a free-piston into rotary motion. For example, Hinds, in U.S. Pat. No. 4,295,381, discloses a free-piston engine with a gyroscopic power transmission. In U.S. Pat. No. 5,850,111, Haaland discloses a push-pull bearing, which allows the free-piston to rotate. In U.S. Pat. No. 6,244,226, Berlinger has disclosed a means for causing a free-piston to rotate. The torque for effecting the free-piston rotation is derived from a plurality of vanes, which cause the piston to rotate upon the piston's movement toward its top dead center position.